The present invention relates to a cruise control apparatus for a vehicle capable of making the vehicle cruise at a constant speed.
In general, a cruise control apparatus for a vehicle is constructed such that a control unit drives, upon receipt of a command signal which is generated by a cruise control switch when it is operated by the driver, a throttle valve in an intake pipe of an internal combustion engine to open and close through the action of an actuator so as to make the vehicle travel at a target speed.
An example of such a cruise control apparatus including a cruise control switch and a control unit is diagrammatically illustrated in FIG. 2. The known cruise control apparatus illustrated includes a cruise control switch 1 and a control unit 2. The cruise control switch 1 is, together with a horn switch 3 and an audio control switch 4, incorporated in a steering wheel (not shown) and constitutes a steering wheel buitl-in switch circuit 5. The horn switch 3 is connected at one end thereof through a slip ring 6 to a horn relay 7 which is in turn connected to a power supply V.sub.1 and a horn 8. The horn switch 3 is also connected at the other end thereof to ground through a bearing in a steering shaft (not shown). The cruise control switch 1 further includes a cancel switch 9 for cancelling a cruise control operation, a set/coast switch 10 for setting a cruise control operation during a non-cruise control operation as well as automatically decelerating the vehicle during a cruise control operation, and a resume/acceleration switch 11 for automatically resuming the vehicle to cruise during a non-cruise control operation as well as accelerating the vehicle during a cruise control operation. The set/coast switch 10 is connected in series with a resistor R.sub.1 to form a first serial connection. The resume/acceleration switch 11 is connected in series with a resistor R.sub.2 to form a second serial connection. The first serial connection including the set/coast switch 10 and the resistor R.sub.1, the second serial connection including the resume/acceleration switch 11 and the resistor R.sub.2, and the cancellation switch 9 are connected in parallel with each other. The cruise control switch 1 is connected at one end thereof to the control unit 2 through a slip ring 12, and at the other end thereof to the horn switch 3 and commonly therewith to ground. The audio control switch 4 is connected at one end thereof to an audio control unit 14 through a slip ring 13, and at the other end thereof to a power supply V.sub.2 through a slip ring 15. Inside the control unit 2, an output of the cruise control switch 1 which is input to the control unit 2 via the slip ring 12 is connected through a resistor R.sub.3 to a power supply V.sub.3 and input to the respective positive terminals of comparators 16 through 18. The negative terminals of the comparators 16 through 18 are connected to the power supply V.sub.3 through corresponding resistors R.sub.4 through R.sub.6, respectively, and at the same time to ground via resistor R.sub.7. The voltages V.sub.H, V.sub.M and V.sub.L are predetermined so as to satisfy the following relationship: V.sub.H &gt;V.sub.M &gt;V.sub.L. The output terminals of the comparators 16 through 18 are connected to a power supply of 5 volts via resistors R.sub.8 through R.sub.10, respectively, and the outputs V.sub.a through V.sub.c of these comparators are input to a central processing unit (CPU) 19.
In operation, when the set/coast switch 10 is turned on with the vehicle travelling under non-cruise control, the input voltage V.sub.i fed to the control unit 2 becomes equal to {R.sub.1 /(R.sub.1 +R.sub.3)}.times.V.sub.3, which is between V.sub.H and V.sub.M, so that the output voltage V.sub.a of the comparator 16 becomes high while the output voltages V.sub.b, V.sub.c remain low. As a result, the CPU 19 is operated to recognize a command signal from the set/coast switch 10, and accordingly drive the throttle valve. Similarly, when the resume/acceleration switch 11 and the cancellation switch 9 are turned on, the CPU 19 also recognizes a command signal therefrom and drives the throttle valve. These operational relationships are tabulated in Table I below.
TABLE I ______________________________________ Output Voltages Control SW Input Voltages V.sub.i V.sub.a V.sub.b V.sub.c ______________________________________ Set/Coast SW {R.sub.1 /(R.sub.1 + R.sub.3)} .times. V.sub.3 0 5 5 Resume/Accel SW {R.sub.2 /(R.sub.2 + R.sub.3)} .times. V.sub.3 0 0 5 Cancellation SW 0 0 0 0 All SWs Off V.sub.3 5 5 5 ______________________________________
With the above-mentioned known cruise control apparatus, however, the switches 9 through 11 are connected to ground commonly with the horn switch 3, so the ground voltage level of these switches varies due to switching on of the horn switch 3, vibrations of the steering system and the like. In addition, since the switches 9 through 11 are grounded through the bearing in the steering shaft, the ground voltage level also varies due to a secular change in the contact resistance of the bearing which gradually increases with time. For these reasons, undesirable situations sometimes take place in which cruise control cannot be set when the set/coast switch 10 is turned on, or cruise control cannot be cancelled when the cancellation switch 9 is turned on.